1. Field of the Invention
The present invention relates to an illumination apparatus and an image reading apparatus suitably applicable particularly to an image scanner, a copying machine, a facsimile machine, or the like that performs image reading by a line sequential method while illuminating the original surface with light.
2. Description of the Related Art
Illumination apparatuses (original illumination apparatuses) using an LED (light emitting diode) or EL (electro luminescence) light source have been known. Such apparatuses have, for example, an array-disposition type construction in which a plurality of LED or EL light sources are arranged along the main scanning direction of the original surface or an edge-disposition type construction in which an LED or EL light source is disposed at an end of a light guide made of a translucent resin or the like with respect to the main scanning direction so that light beams emitted from the light source propagate through the light guide. Nowadays, with increases in the light emission efficiency of LED and EL light sources, the edge-disposition type arrangement, which allows a reduction in the number of LEDs or a reduction in the EL light emission area, is receiving attention.
Some originals read by an image reading apparatus are placed on the original platen glass with a portion thereof, such as the binding portion of a book, floating. It is necessary to illuminate such a floating portion without a significant difference in the light intensity from that in a case where the original surface is placed on the original platen glass. To meet such need, methods of improving illumination depth characteristics (light intensity variation with floating of the original) have been developed.
U.S. Patent Application Publication No. 2009/0303732 discloses a technique in which a deflection means is provided in the exit portion of the light guide in the edge-disposition type arrangement to shift the light intensity distribution of the emitted light along the sub-scanning direction, thereby improving illumination depth characteristics. Specifically, the exit portion has first flat surface that causes emitted light to illuminate a normal original surface position and a second flat surface that is inclined relative to the first flat surface and causes emitted light to illuminate the position of a floating portion. U.S. Patent Application Publication. No. 2011/0149590 discloses a technique in which a diffusive, reflective member is provided along the main scanning direction outside the lower region of the light guide in the edge-disposition type arrangement with an air layer therebetween to enhance the diffusing effect, thereby improving illumination depth characteristics.
However, with increasing needs for improved image quality and increased processing speed in the image reading apparatus, further improvement meeting such needs is required nowadays. The apparatus disclosed in U.S. Patent Application Publication No. 2009/0303732 cannot have an illumination area with stable light intensity that is sufficiently large along the sub-scanning direction to achieve high image quality. Specifically, to achieve high-quality image reading, a reading system using a reduction optical system having a long optical path length and a large depth of field is typically adopted. This system needs a countermeasure against a large variation in the reading position caused by a small difference in the mounting position and the mounting angle of a mirror and an imaging optical system arranged in the optical path.
On the other hand, the apparatus disclosed in U.S. Patent Application Publication No. 2009/0303732 uses a CIS (contact image sensor) that reads images with a unity (or one-to-one) magnification imaging system having a short optical path length. Therefore, large variations in the reading position with respect to the sub-scanning direction are not taken into consideration.
Although U.S. Patent Application Publication No. 2011/0149590 provides improvement in the illumination depth by ensuring an illumination area with stable light intensity that is sufficiently large along the sub-scanning direction by virtue of a diffusive member for enhancing the diffusing effect, no consideration is given to improvement in the processing speed. In other words, in the apparatus disclosed in U.S. Patent Application Publication No. 2011/0149590, a decrease in the light intensity occurs due to the diffusing effect, leading to insufficient light intensity on the original surface.